Electroconductive paper

ABSTRACT

A cellulosic paper sheet, suitable for use as a substrate for receiving a coating of a light-sensitive electroconductive material such as zinc oxide, which paper sheet comprises a coating thereon of from 0.5 to 5 lbs. per ream of colloidal chrysolite asbestos and a quantity of lithium chloride therein to impart a surface resistivity of less than 1010 ohms at a relative humidity of 15 percent, while maintaining a volume resistivity of greater than 1010 ohms under the same environmental conditions.

United States Patent 1,532,033 5/1925 Shaw lnventors Appl. No.

Filed Patented Assignee Robert A. Cross Waltham;

John F. Hegarty, Somerville, both of Mass. 788,087

Dec. 30, 1968 Sept. 21,1971

Amicon Corporation Lexington, Mass.

ELECTROCONDUCTIVE PAPER 10 Claims, No Drawings U.S. Cl 117/201, 96/l.8,96/85, 117/154, 162/129, 252/518, 260/37 Int. Cl B44d 1/18 Field ofSearch 1 17/201,

References Cited UNITED STATES PATENTS Primary Examiner-Ralph S. KendallAttorney-R. W. Furlong ABSTRACT: A cellulosic paper sheet, suitable foruse as a substrate for receiving a coating of a light-sensitiveelectroconductive material such as zinc oxide, which paper sheetcomprises a coating thereon of from 0,5 to 5 lbs. per ream of colloidalchrysolite asbestos and a quantity of lithium chloride therein to imparta surface resistivity of less than 10 ohms at a relative humidity of 15percent, while maintaining a volume resistivity of greater than l0' ohmsunder the same environmental conditions.

ELECTROCONDUCTIVE PAPER BACKGROUND OF THE INVENTION It has long been aproblem in the art to provide conductive papers which, when coated withsuch light-sensitive electroconductive materials as zinc oxide, providepapers suitable for use in electrostatic printing operations. Byconductive paper" is meant a paper with a surface resistivity of from to10" ohms in an environment of from 90 to 12 percent relative humidity.The importance of maintaining the desired conductivity over a wide rangeof humidity is particularly important. Treating a paper withglycerine-based humectants has not been completely satisfactory becausein places having low relative humidity, or under high-temperaturestorage conditions, there is a tendency for such papers to drift fromthe desired range of resistivity. Moreover, at high relative humidities,such papers become rather limp and have inferior handling qualities.

The prior art has recognized the value of using a colloidal chrysoliteasbestos to impart conductivity to cellulosic paper. More particularly,Willard E. Carlsons U.S. Pat. No. 3,337,392 discloses the use of such anasbestos and polyvalent metal salts in the formation of a conductivepaper matrix. However, it has now been discovered that the optimumadvantage in utilizing colloidal chrysolite asbestos is to be obtainedin avoiding the dispersal of the material throughout the paper web, i.e.in restricting the presence of the asbestos to the surface of the paper.A distribution of the conductivity-imparting material in the substratestock is undesirable for a number of reasons. For example, this mode ofemploying the conductivity-imparting material increases the likelihoodof strike-through" of the material which interferes with image formationon the underlying sheets in a pad of sheets. Also, confining a givenquantity of conductivity-imparting material to the surface of a sheet ofsubstrate paper to yield high surface conductivity results in betterimage formation than when the same quantity of material is dispersedthrough the bulk of the sheet which dispersal, while elevating the bulkconductivi ty, reduces the surface conductivity.

Moreover, such paper as disclosed by Carlson leaves much to be desiredat low relative humidities, i.e. those in the 12 to percent range.

Other approaches used in the prior art are those utilizing polymersbearing quaternary ammonium groups. These materials are particularlygood, probably because they are readily retained on the paper surface.Still, such polymers are expensive and, are also under some conditions,undesirably odorous.

SUMMARY OF THE INVENTION Therefore, it is an object of the invention toprovide means for achieving a coating composition comprising colloidalchrysolite asbestos which coating can be retained on the surface of apaper, thereby contributing to avoiding the aforementioned problem, andto obtaining of a highly satisfactory surface-conductive paper for useas a substrate for papers used in electrostatic-printing processes overa wide range of humidity levels.

Another object of the invention is to provide novel compositions forforming such coatings.

A further object of the invention is to provide a paper suitable forstorage and use at relative humidities as low as from about 12 percent.

Other objects of the invention will be obvious to those skilled in theart on reading the instant specification.

The above-listed objects have been substantially accomplished by coatinga suitable paper substrate with a composition comprising defibrillatedcolloidal asbestos in such a concentration that it forms a gel structurewithin the coating composition thereby (l) entrapping any otherleachable components of the composition, and (2) providing a coatingthat will have excellent holdout," i.e. a substrate coating that of fersa substantial resistance to the penetration of solvent used in thecopy-making process. Among the more important optional ingredients ofthe coating that will be retained are the hygroscopic salts such ascalcium chloride, lithium chloride, etc. Indeed a particular advantageof the invention is that it provides an excellent means for retainingthe monovalent lithium chloride on the surface of the paper to betreated and thereby means for providing an excellent low-humidity paper.Operable coating compositions according to the invention can be achievedwith salt: asbestos ratios of up to 1:1. The coating compositions arenormally applied while suspended at from 1 part to 10 parts solids per1100 parts of water. The coating process is carried out so that anultimate coating of from 0.5 to 5 lbs. of asbestos per ream of paper isachieved.

The colloidal chrysolite asbestos preferred for use in the invention isthat known as Coalinga asbestos.

Asbestos is a widely available and relatively inexpensive materialhaving certain desirable physical properties which make it very usefulas a filler in various chemical composi tions. However, in order toachieve maximum effectiveness of the asbestos in improving properties:of chemical compositions, it is necessary to reduce it to the form ofprimary fibrils.

A particular chrysolite asbestos material preferred in the presentinvention is obtained from a deposit located near Coalinga, California.This is a newly found deposit believed to be the largest single asbestosdeposit in the world. Asbestos mined at this location is short-fiberedchrysolite asbestos and would be classified between grade 5 and grade 7according to the Canadian Standards Classification. Coalinga asbestos,however, has a much more uniform distribution of individual fiberlengths and diameters than do comparable grades of Canadian chrysoliteasbestos. In addition, the surface area of Coalinga asbestos issubstantially greater than that of the Canadian type of asbestos.Refined asbestos obtained from the Coalinga deposit has properties whichare substantially different from any prior art asbestos. While notwishing to be limited to the geographic deposit at Coalinga, California,the type of asbestos recovered from ore such as found there is preferredin the present invention.

It is surprising that a coating of Coalinga asbestos provides reallygood electrostatic-printing paper over a rather wide range of humiditieswithout the addition of salt. However, to achieve the broader advantagesof the invention, some salt and/or polyelectrolyte complex resin isadvantageously included in the coating formulation.

The most advantageous salt used in the composition of the invention isthe monovalent lithium chloride not only because it provides excellentlow-humidity moisture retention properties, but also because it isretained in the asbestos-gel coating to a surprising extent. Otherhygroscopic salts may also be included in the composition of theinvention. Among some such salts are calcium chloride, sodium bromide,potassium nitrate, aluminum chloride and the like.

The polyelectrolyte complex resins useful in the process of theinvention include those such as described in an article entitledPolyelectrolyte Complexes" by Alan S. Michaels in the Oct. 1965 issue ofIndustrial and Engineering Chemistry, a publication of the AmericanChemical Society. The polyelectrolyte complex resin formed by the ionicassociation of a sul fonate polyanion such as poly(sodium styrenesulfonate) and a quaternary ammoniurn'type polycation such as poly(vinylbenzyltrimethyl ammonium chloride).

It has been found that such resins when incorporated into compositionsaccording to the invention, provide a relatively high increase infilm'forming properties with a relatively low drop in conductivity.Quantities of up to 0.5 part of polyelectrolyte complex resin per partof asbestos are suitable for use in the coating compositions of theinvention.

Adjuvants such as pigments, thickening aids, inert fillers and othersuch materials known to the art may be added to these formulationsinsofar as they do not upset the balance of electroconductive propertieswhich it is the purpose of this invention to achieve. However, theconductivity-imparting portion of the composition can consistessentially of the asbestos,

salt and/or polyelectrolyte complex resin as described hereinabove.

Water will, of course, usually be the convenient vehicle for use in thecoating operation whether asbestos alone, asbestos and salt, and/or apolyelectrolyte complex resin are used in the process of the invention.

ILLUSTRATIVE EXAMPLE OF THE INVENTION Example 1 A chrysolite asbestos ofthe Coalinga type sold by Union Carbide Corporation was dispersed inwater to form a slurry containing 5 percent of the asbestos. The pH ofthe slurry was lowered to about 4.3 with a small quantity of aceticacid. Thereupon the pH of the slurry was raised to 5.5 by adding a smallquantity of barium hydroxide thereto. Then a single drop of a 31 percentaqueous solution of poly(vinylbenzyl trimethyl arnmoniurn chloride)wasadded tothe slurry.

Separately, a dispersion of 325-mesh anionic polyelectrolyte complexresin of the type sold under the trade designation Ioplex by AmiconCorporation was also dispersed to form a 5 percent solids suspension inwater.

These two dispersions, i.e. the asbestos and resin dispersions, weremixed in quantities so that the resultant dispersion contained fiveparts asbestos and one part polyelectrolyte complex resin. Thereuponenough lithium chloride was dissolved in the slurry so that the resinto-salt ratio was 1 to 1.

The slurry was drawn down onto a sheet of Crocker Videograph paper usinga No. 18 wire-wound casting rod of the type well known in thepaper-coating art. The resulting coating weight, on a dry basis, wasabout 1.7 lbs per ream of paper. This coated paper was dried for 5minutes in a circulating air oven at 80 C. A smooth, colorless, coatingwas observed on the dried sheet.

Conductivity of the paper was tested by placing the sheet in a dry boxhaving a relative humidity of 16 percent. After conditioning the paperfor about 16 hours in this dry environment, its surface and volumeresistivity were tested with a 6105 Resistivity Adapter sold by KeithleyInstruments Co. and a Keithley 621 Electrometer using a Sorenson HighVoltage Power Supply, respectively. Surface resistivity was 2 l0 ohm/sq.and volume resistivity was 10 ohm-cm.

These values indicated that there was little or no strikethrough of thecoating composition below the surface of the paper.

Example 2 The procedure of example 1 was repeated excepting that thepolyelectrolyte complex resin was omitted from the papercoatingcomposition. 7

The surface resistivity of the resulting paper was 5 l0 ohms/sq.

Example 3 The procedure of example 1 was again repeated excepting that(1) both the polyelectrolyte complex resin and the lithium chloride saltwere omitted from the formulation; (2) the coating contained 2.3 lbs perream of asbestos; and (3) the testing was carried out at 45 percentrelative humidity.

Surface resistivity was 1X10".

What is claimed is:

1. A conductive cellulosic paper sheet comprising a paper substrate anda coating thereon, said coating comprising a colloidal chrysoliteasbestos in the quantity of from 0.5 to 5 lbs. per ream of paper sheet,said resultant sheet having a surface resistivity of less than 10 ohmsat a relative humidity of 15 percent and a volume resistivity of morethan about 10 ohmcm. at 15 percent relative humidity.

2. A conductive sheet as described in claim 1 which comprises ahygroscopic salt as a component of said coating in a quantity such thata salt-to-absetos ratio of up to about 1 to l is achieved.

3. A conductive sheet as described in claim 2 wherein said hygroscopicsalt is lithium chloride.

4. A conductive sheet as described in claim 1 wherein said coatingcomprises a polyelectrolyte complex resin in a quantity such that aresin-to-asbestos ratio of up to about 0.5 to l is achieved.

5. A conductive sheet as defined in claim 4 wherein said polyelectrolytecomplex resin is of the type formed by the ionic cross linking of asulfonate-type polyanion and a quaternary ammonium-type polycation.

6. A process for imparting electroconductivity to a sheet of paper athumidities as low as 12 percent comprising the steps of:

1. forming an aqueous coating composition including at least 1 partcolloidal chrysolite asbestos per parts of water,

2. coating said composition on said paper sheet to deposit, on thesurface thereof, a gel network comprising from 0.5 to 5 lbs. of asbestosper ream of paper, and

3. drying said paper sheet, thereby forming a paper of the desiredelectroconductive character.

7. A process as defined in claim 6 wherein a polyelectrolyte complexresin is included in said coating composition.

8. A process as defined in claim 7 wherein a hygroscopic salt isincluded in said coating composition.

9. A coating composition, suitable for use in imparting conductivity topaper at low humidity levels, said composition consisting essentiallyof:

l. 10 parts of colloidal chrysolite asbestos,

2. 2 to 10 parts of lithium chloride,

3. O to 5 parts of a polyelectrolyte complex resin, and

4. a quantity of water in which to disperse the foregoing ingredients.

10. The coating composition of claim 9 including at least 2 parts of apolyelectrolyte complex resin.

P0-' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3.607377 Dated September 21, 1971 fls) Robert A. Cross and John F.Hegarty It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 21, "chrysolite" should be spelled --chrysotile--;

Column 1, line 26, "chrysolite" should be spelled --chrysotile--;

Column 1, line 55, "chrysolite" should bespelled --chrysotile--;

Column 2, line 1 4, "chrysolite" should be spelled --chrysotile--;

Column 2, line 23, "chrysolite" should be spelled --chrysotile--;

Column 2, line 27, "chr'ysolite" should be spelled ---chrysotile---;

Column 2, line 32, "chrysolite" should be spelled --chrysotile--;

Column 3, line '11, "chrysolite" should be spelled --chrysotile--;

Column 0, claim 1, line 11, "chrysolite" should be spelled--chrys0tile--;

Column 4, claim 2, line 18, "asbestos" is misspelled;

Column 1, claim 6, line 3'4, "chrysolite" should be spelled--chrysotile--;

Column l, claim 9, line 18, "chrysolite" should be spelled--chrysotile--.

Signed and sealed this 21st day of March 1972.

(SEAL) Attest:

EDWARD M,FLEHER,JR. ROBERT GOTTSCHALK Attesting 0f lcer Commissioner F De n HTHI we M-

2. A conductive sheet as described in claim 1 which comprises ahygroscopic salt as a component of said coating in a quantity such thata salt-to-absetos ratio of up to about 1 to 1 is achieved.
 2. coatingsaid composition on said paper sheet to deposit, on the surface thereof,a gel network comprising from 0.5 to 5 lbs. of asbestos per ream ofpaper, and
 2. 2 to 10 parts of lithium chloride,
 3. 0 to 5 parts of apolyelectrolyte complex resin, and
 3. drying said paper sheet, therebyforming a paper of the desired electroconductive character.
 3. Aconductive sheet as described in claim 2 wherein said hygroscopic saltis lithium chloride.
 4. A conductive sheet as described in claim 1wherein said coating comprises a polyelectrolyte complex resin in aquantity such that a resin-to-asbestos ratio of up to about 0.5 to 1 isachieved.
 4. a quantity of water in which to disperse the foregoingingredients.
 5. A conductive sheet as defined in claim 4 wherein saidpolyelectrolyte complex resin is of the type formed by the ionic crosslinking of a sulfonate-type polyanion and a quaternary ammonium-typepolycation.
 6. A process for imparting electroconductivity to a sheet ofpaper at humidities as low as 12 percent comprising the steps of:
 7. Aprocess as defined in claim 6 wherein a polyelectrolyte complex resin isincluded in said coating composition.
 8. A process as defined in claim 7wherein a hygroscopic salt is included in said coating composition.
 9. Acoating composition, suitable for use in imparting conductivity to paperat low humidity levels, said composition consisting essentially of: 10.The coating composition of claim 9 including at least 2 parts of apolyelectrolyte complex resin.